More missing calls to Result::ok() in libbpf_android tests

Test: m checkbuild
Change-Id: Idb70b975e3c3bcbaa78cb493ec8f66b9b625ef74
diff --git a/libbpf_android/include/bpf/BpfMap.h b/libbpf_android/include/bpf/BpfMap.h
index bdc4cfd..4310f56 100644
--- a/libbpf_android/include/bpf/BpfMap.h
+++ b/libbpf_android/include/bpf/BpfMap.h
@@ -156,7 +156,7 @@
 
     base::Result<bool> isEmpty() const {
         auto key = this->getFirstKey();
-        if (!key) {
+        if (!key.ok()) {
             // Return error code ENOENT means the map is empty
             if (key.error().code() == ENOENT) return true;
             return key.error();